JP 2010-100077A (Patent Literature 1) discloses a vehicle seat sliding apparatus in the related art. As is shown in FIG. 9, the vehicle seat sliding apparatus 100 includes a lower rail 110 fixed to a vehicle floor and an upper rail 120 fixed to a seat and connected to the lower rail 110 in a relatively movable manner.
The lower rail 110 has a bottom wall 111, a pair of vertical walls 112 extending upward from both ends of the bottom wall 111 in a width direction, and a pair of flanges 113 projecting from upper ends of the vertical walls 112 inwardly in the width direction in which the flanges face each other and further bending toward lower ends of the vertical walls 112. A plurality of locking claws 113a are provided side by side in a direction of the relative movement (a direction perpendicular to the sheet surface of FIG. 9) at tip ends of the respective flanges 113.
On the other hand, the upper rail 120 has a top wall 121 fixed to a lower surface of the seat, a pair of vertical walls 122 extending downward from both ends of the top wall 121 in the width direction, a pair of outward extending wall portions 123 extending outward in the width direction diagonally from lower ends of the vertical walls 122, and a pair of extending wall portions 124 extending in a top-bottom direction from outer ends of the outward extending wall portions 123 in the width direction so as to be surrounded by the bottom wall 111 and the flanges 113. At a predetermined position in the direction of the relative movement, the upper rail 120 is provided with through-holes 125 in lower end portions of the respective vertical walls 122 and the respective extending wall portions 124 including the respective outward extending wall portions 123 entirely. In short, the upper rail 120 is opened downward by the through-holes 125.
A lock lever 130 connected to the upper rail 120 in a rotationally movable manner about an axis line extending in the width direction is disposed in a space defined between the lower rail 110 and the upper rail 120. The lock lever 130 has a lock lever 131 allowed to pass through the through-holes 125 in the width direction. When the lock lever 130 is biased by a biasing member (not shown), the lock plate 131 rises and the locking claws 113a of the lower rail 110 fit into locking portions 131a in the shape of rectangular holes provided to the lock plate 131. A movement of the upper rail 120 with respect to the lower rail 110 is thus locked. Contrarily, the locking of the movement of the upper rail 120 with respect to the lower rail 110 is unlocked when the lock plate 131 comes down against a biasing force of the biasing member by an operation force of an operation member (not shown) so that the locking claws 113a come off the locking portions 131a. 
In Patent Literature 1, however, the through-holes 125 provided to the upper rail 120 inevitably deteriorate strength of the upper rail 120. Such deterioration of strength of the upper rail 120 becomes more noticeable with an increase of the opening area of the through-holes 125. It is therefore preferable to reduce the opening area of the through-holes 125. On the other hand, when the opening area of the through-holes 125 is reduced, a rotationally movable range (stroke) of the lock lever 130 is reduced, too, and a selective locking operation of a relative movement of the lower rail 110 and the upper rail 120 by the lock lever 130 may possibly become unstable.